Electrostatic chuck

ABSTRACT

The object of the present invention is to provide an electrostatic chuck in which the surface can be kept smooth after being exposed to plasma, so as to protect a material to be clamped such as a silicon wafer from being contaminated with particles, and which is excellent in clamping and releasing a material to be clamped. According to the present invention, there is provided an electrostatic chuck comprising a dielectric material in which alumina is 99.4 wt % or more, titanium oxide is more than 0.2 wt % and equal to or less than 0.6 wt %, whose average particle diameter is 2 μm or less, and whose volume resistivity is 10 8 -10 11  Ωcm in room temperature, wherein the electrostatic chuck is used in a low temperature of 100° C. or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrostatic chuck for clamping andfixing a material to be clamped such as a semiconductor wafer or a glasssubstrate for an FPD with electrostatic force.

2. Description of Prior Art

Conventionally, a ceramic dielectric material of an electrostatic chuckis provided for the purpose of controlling the electricalcharacteristics (See Document 1).

However, if the ceramic structure is exposed to a plasma atmosphere, thestructure is subject to corrosion, and the surface roughness isdeteriorated. Consequently, there are some cases where variations in thecontact state between the surface of the electrostatic chuck and thewafer occur over time, or there are other cases where grains aredisjoined from a sintered body and the disjoined grain particles causethe wiring of an LSI circuit to be shorted.

In another conventional example, an alumina ceramic material having aparticle diameter of 2 μm or less and a relative density of 99.9% inwhich the plasma-resistance is improved is applied to an electrostaticchuck (See Document 2). However, in this example, even if theplasma-resistance is improved, there is no description of the electricalcharacteristics, and it is impossible to perform fundamental functionsof a Johnsen-Rahbeck electrostatic chuck which enables great clampingforce.

Also, an alunina ceramic which contains titanium oxide of 0.1-1 wt % andhas volume resistivity of 10⁰-10⁴ Ωcm has been proposed (See Document3). However, in this case, it is impossible to obtain electricalcharacteristics for performing functions of an electrostatic chuck.

Also, an electrostatic chuck in which the volume resistivity is reducedby adding titanium oxide of 0.5-2 wt % to an alumina ceramic has beenproposed (See Document 4). This document has disclosed that theresistivity is not reduced in a case of less than 0.5 wt %, and too muchcurrent flows in a case of 2.0 wt % or more. It has also disclosed thattitanium oxide is precipitated in the grain boundary of aluminaceramics. Although an additive of at least 0.5 wt % is required toreduce the volume resistivity, such an amount of the additive is toomuch for an electrostatic chuck which requires a strict limitation incontamination with respect to a material to be clamped.

Also, there has been proposed an electrostatic chuck in which alumina of99% or more is contained, the average particle diameter is 1-3 μm, andthe volume resistivity is 10⁸-10¹¹ Ωcm in a temperature of 300-500° C.(See Document 5). However, there is no description of properties of andielectric material required for an electrostatic chuck which is used inanother temperature range, for example, a relatively low temperaturesuch as 100° C. or less.

Document 1: Japanese Patent No. 3084869

Document 2: Japanese Patent Application Publication No. 10-279349

Document 3: Japanese Patent Application Publication No. 2004-18296

Document 4: Japanese Pre-grant Publication No. 6-97675

Document 5: Japanese Patent Application Publication No. 11-312729

The object of the present invention is to provide an electrostatic chuckin which the surface can be kept smooth after being exposed to plasma,so as to protect a material to be clamped such as a silicon wafer frombeing contaminated with particles, and which is excellent in clampingand releasing a material to be clamped.

SUMMARY OF THE INVENTION

In order to achieve the above-mentioned object, according to the presentinvention, there is provided an electrostatic chuck including adielectric material in which alumina is 99.4 wt % or more, titaniumoxide is more than 0.2 wt % and equal to or less than 0.6 wt %, whoseaverage particle diameter is 2 μm or less, and whose volume resistivityis 10⁸-10¹¹ Ωcm in a room temperature, wherein the electrostatic chuckis used in a low temperature of 100° C. or less. With this, it ispossible to improve the plasma resistance of the electrostatic chuckdielectric material and achieve fundamental functions of theelectrostatic chuck at the same time.

The reason why the volume resistivity needs to be 10⁸-10¹¹ Ωcm is forusing Johnsen-Rahbeck effects as clamping force of the electrostaticchuck. It is possible to generate very great clamping force by usingJohnsen-Rahbeck effects. Consequently, it is possible to reduce thecontact area with a material to be clamped so as to be 1-10% withrespect to the area of the clamping surface by providing protrusions onthe surface of the electrostatic chuck.

Further, by adjusting the height of the protrusions provided on thesurface to be 5-15 μm, it is possible to exert clamping force even innon-contact portions. As a result of this, the area of the protrusionscan be 0.001% or more and less than 0.5% with respect to the area of theclamping surface. As for the temperature of the material to be clamped,heat transfer is performed via the contact portions. Consequently, evenif the structure of the protrusions is subject to corrosion caused bythe plasma, such effect can be reduced as the contact area of theprotrusions decreases. Therefore, it is possible to achieve anelectrostatic chuck in which little variations occur over time byimproving the plasma-resistance and reducing the contact area with thematerial to be clamped.

In order to improve the response properties of the clamping force, it isnecessary to decrease the value of the following equation:ts=1.731×10⁴¹×ρ(εr+d/h) (sec.)where ts is time (sec.) required for decreasing the clamping force to be2% with respect to the initial clamping force of 100%, ρ is volumeresistivity of the dielectric layer (Ωm), εr is the relative dielectricconstant of the dielectric layer, d is the thickness of the dielectriclayer (m), and h is the height of the protrusions (m). When the value ofthis equation is 0.001-0.6 and the height of the protrusions is 5-15 μm,the area of the protrusions can be 0.001-0.5% with respect to theclamping surface. At the same time, it is possible to achieve anelectrostatic chuck having good response of clamping force with respectto applying or removing voltage.

The above-mentioned equation is obtained by making an analyticalcalculation based on the equivalent circuit shown in FIG. 1 and derivingEquations 1-4. In these equations, q₁ is the charge density, S is theelectrode area, C is the capacitance, G is the conductance, V is theapplied voltage, t is time (variable), and T is time for applyingvoltage. $\begin{matrix}{{f = {\frac{q_{1}^{2}}{2\quad\delta_{0}}\quad\left( {q_{0} = \frac{Q_{1}}{S}} \right)}}{0 \leq t \leq T}} & {{Equation}\quad 1} \\{{q_{1}(t)} = {{q_{r}(t)} + {q_{j}(t)}}} & (1) \\{q_{r} = {C_{1}\frac{C_{2}}{C_{1} + C_{2}}{V \cdot {\exp\left( {{- \frac{G_{2}}{C_{1} + C_{2}}}t} \right)}}}} & (2) \\{{q_{j} = {C_{1}\frac{G_{2}}{G_{1} + G_{2}}{V \cdot \left\{ {1 - {\exp\left( {{- \frac{G_{2}}{C_{1} + C_{2}}}t} \right)}} \right\}}}}{t > T}} & (3) \\{{q_{1}(t)} = {{q_{r}(t)} + {q_{j}(t)} - \left\{ {{q_{r}\left( {t - T} \right)} + {q_{j}\left( {t - T} \right)}} \right\}}} & (4)\end{matrix}$ $\begin{matrix}{\tau = \frac{2\left( {C_{1} + C_{2}} \right)}{G_{2}}} & {{Equation}\quad 2} \\{{C_{1} = {ɛ_{0}\frac{S}{d}}}{C_{2} = {ɛ_{0}ɛ_{r}\frac{S}{h}}}{G_{2} = {\frac{1}{R_{2}} = \frac{S}{\rho\quad d}}}} & {{Equation}\quad 3}\end{matrix}$ $\begin{matrix}{{t > T}{q_{1} = {C_{1}\frac{C_{1}}{C_{1} + C_{2}}{V \cdot {\exp\left( {{- \frac{G_{2}}{C_{1} + C_{2}}}\left( {t - T} \right)} \right)}}}}} & {{Equation}\quad 4}\end{matrix}$

According to another aspect of the present invention, there is providedan electrostatic chuck including a dielectric material in which aluminais 99.4 wt % or more, titanium oxide is more than 0.2 wt % and equal toor less than 0.6 wt %, whose average particle diameter is 2 μm or less,whose bulk density is 3.97 g/cm³ or more, and whose volume resistivityis 10⁸-10¹¹ Ωcm in room temperature, wherein the electrostatic chuck isused in a low temperature of 100° C. or less. With this, the porosity ofthe structure of the electrostatic chuck is small, and improvement ofthe plasma resistance and fundamental functions of the electrostaticchuck can be achieved at the same time.

In the above-mentioned electrostatic chuck, the dielectric materialhaving a smooth surface includes a plurality of protrusions on which amaterial to be clamped is mounted, in which the ratio between the totalarea of the top surfaces of the protrusions and the area of the surfaceof the dielectric material is equal to or more than 0.001% and less than0.5%, and the height of the protrusions is 5-15 μm. With this, it ispossible to minimize the effect of variation in the clamping statecaused by damage of the surface of the contact area with the material tobe clamped due to corrosion by plasma. However, if the ratio of thecontact area is less than 0.001%, the size of each protrusion will betoo small and the processing will be difficult. If the ratio of thecontact area exceeds 1%, it becomes impossible to disregard the effectof plasma corrosion to the surface of the protrusions in contact withthe material to be clamped.

BRIEF DESCRIEPTION OF THE DRAWINGS

FIG. 1 shows an electrostatic chuck according to the present invention;

FIG. 2 shows an equivalent circuit of the electrostatic chuck accordingto the present invention; and

FIG. 3 is an enlarged view of the surface pattern of the electrostaticchuck according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Alumina, titanium oxide, and transition metal oxide as ingredients weregranulated at a mixing ratio shown in Table 1. The average particlediameter of the alumina was 0.1 μm, and the purity of the alumina was99.99% or more. The purity of the titanium oxide was 98% or more.

Slurry Preparation, Granulation, and Raw Processing

The above ingredients were mixed at the mixing ratio shown in Table 1,and crushed. After an acrylic binder was added and adjusted, granulationby spray-drying was performed to prepare granulated powder. After thegranulated powder was filled in a rubber mold, CIP (pressure: 1 ton/cm²)was performed, and an ingot was formed. The ingot was processed to havea predetermined shape, and a raw formed body was obtained. Ion-exchangewater or the like was used for mixing, so that contaminants could beprevented from entering as much as possible.

Firing

The above raw formed body was fired in an atmosphere of nitrogen andhydrogen gas. The firing temperature was 1150-1350° C., and the firingtime was 1-8 hours. That is, conditions for achieving the highest bulkdensity were selected. In this instance, humidifying gas was used so asto degrease. By performing reduction firing, it is possible to achievenon-stoichiometric composition of titanium oxide and adjustment of thevolume resistivity.

HMP Processing

Further, HEP processing was performed. The conditions for HIP were Argas of 1500 atm. and a temperature identical to the firing temperatureor less than the firing temperature by 30° C.

Measurements of Properties

After the HEP processing, measurements of the bulk density, the averageparticle diameter by observing of the structure of the fired body withan SEM, the volume resistivity, the friction force in vacuum, and theremaining time were performed. For the measurements of the frictionforce and the remaining time, the thickness of the ceramic dielectriclayer was 1 mm. Voltage of 200 V was applied as clamping voltage. Forthe measurement of the remaining time, the power supply was turned offafter voltage was applied for 1 minute, and the damping of the remainingfriction force was measured. The material to be clamped was a mirrorsurface of a silicon wafer. The remaining time was a period of time fromtuning the power supply off to damping the friction force to 2%,

Also, plasma irradiation was actually performed, and variation in thesurface roughness (centerline average roughness Ra) of the ceramics wasmeasured. The surface roughness Ra was 0.05 μm or less in the initialstate. Plasma was discharged for 5 hours at 1000 W in a reactive ionetching apparatus with CF₄+O₂ as etching gas.

Further, in order to evaluate actual clamping force of the electrostaticchuck, pressure of He gas was applied between some of the samples andthe material to be clamped so as to measure the pressure where thematerial to be clamped is released (POPOFF clamping force). The clampingvoltage was 1000 V.

Comparative Products

Examples of alumina ceramics according to the conventional method wereprepared for comparison. The composition of comparative product 1 wasalumina of 98 wt % having an average particle diameter of 5 μm andtitanium oxide of 2 wt %. The composition of comparative product 2 wasalumina of 99 wt % and titanium oxide of 1 wt %, and the firingtemperature was 1580° C. The surface roughness of comparative product 1was Ra 0.23 μm in the initial state. The surface roughness ofcomparative product 2 was Ra 0.2 μm in the initial state. No HIPprocessing was performed to comparative products 1 and 2.

The results of the above-mentioned tests are shown in Tables 1 and 2. Itturned out that a structure having an average particle diameter of 2 μmor less and volume resistivity enough to function as an electrostaticchuck can be obtained in an addition amount of titanium oxide which ismore than 0.2 wt % and equal to and less than 0.6 wt % by controllingthe firing temperature. Specifically, the same effect can be obtained inan extremely small addition amount compared to the amount of aconventional case where the particle diameter is 50 μm or more. It isassumed that the reason is because titanium oxide is easily dissolvedinto alumina when the particle diameter is reduced. It is also assumedthat chemical contamination to a silicon wafer or the like can becontrolled significantly compared to the conventional art because theaddition amount of titanium oxide can be reduced by reducing theparticle diameter of the sintered body. TABLE 1 Firing Titaniumtemperature Bulk density of No. Alumina oxide ° C. fired body g/cm³ 1 100 wt %   0 wt % 1240 3.79 2  100 wt %   0 wt % 1270 3.88 3 99.9 wt %0.1 wt % 1300 3.78 4 99.9 wt % 0.1 wt % 1240 3.89 5 99.8 wt % 0.2 wt %1210 3.74 6 99.8 wt % 0.2 wt % 1240 3.89 7 99.7 wt % 0.3 wt % 1180 3.238 99.7 wt % 0.3 wt % 1210 3.91 9 99.6 wt % 0.4 wt % 1180 3.60 10  99.6wt % 0.4 wt % 1210 3.92 11  99.5 wt % 0.5 wt % 1150 3.60 12  99.5 wt %0.5 wt % 1180 3.92 13  99.4 wt % 0.6 wt % 1150 3.92 14  99.4 wt % 0.6 wt% 1180 3.92 Comparative   98 wt %   2 wt % 1580 3.75 product 1Comparative   99 wt %   1 wt % 1580 3.7 product 2

TABLE 2 Average particle Volume Friction force Surface roughness Surfaceroughness Bulk density diameter of fired resistivity at voltage ofRemaining before plasma after plasma after HIP body after HIP 200 V timeprocessing processing No. g/cm³ μm Ωcm gf/cm³ Min. μm μm 2 3.98 0.9>10¹⁵   >400 >300 0.03 0.06 4 3.98 1.1 10¹⁵  >400 >300 0.03 0.06 6 3.981.3  10^(12.7) >400 120 0.03 0.06 8 3.98 1.4 10¹⁰  >400 8 0.03 0.06 10 3.98 1.5 10^(9.3) >400 4 0.03 0.07 12  3.98 1.5 10^(8.5) >400 1 0.030.07 14  3.97 1.7 10^(8.3) >400 1 0.03 0.07 Comparative — 80 10^(10.3) >400 15 0.23 0.56 product 1 Comparative — 70 10¹¹  >400 300.2 0.6 product 2

As a result of the electrical properties evaluation, it turned out thatthe volume resistivity can be controlled in the wide range of 10⁸-10¹⁶Ωcm depending on the addition amount of titanium oxide alone or titaniumoxide and transition metal oxide.

Regarding the electrical properties required for a dielectric materialfor an electrostatic chuck, it is preferable that the volume resistivityis 10⁸-10¹¹ Ωcm in a temperature where the electrostatic chuck is used.If it is less than 10⁸ Ωcm which is the minimum, too much current flowsinto a wafer and the device might be damaged. If it is than 10¹¹ Ωcmwhich is the maximum, the response of wafer clamping and release to theapplied voltage is deteriorated.

In processes such as an etching process of 100° C. or less, for example,the minimum value is preferably around 10⁹-10¹¹ Ωcm.

Also, if titanium oxide is more than 0.6 wt %, the volume resistivitybecomes less than 10⁸ Ωcm, too much current flows into a wafer and thedevice might be damaged. If titanium oxide is 0.2 wt % or less, theeffect of reducing the volume resistivity by adding titanium oxide mightbe reduced.

The plasma resistance was evaluated based on variation in the surfaceroughness because any material is etched if ion energy in plasma isexcessive.

As for the ceramic dielectric material according to the presentinvention, variation in the surface roughness was significantly smallcompared to a conventional one. It is assumed that this is because thesize of particles is small.

An electrostatic chuck comprising a dielectric material having a smoothsurface was produced, in which a plurality of protrusions on which amaterial to be clamped is mounted were formed and the volume resistivitywas 10⁹³ Ωcm, and in which the ratio between the total area of the topsurfaces of the protrusions and the area of the surface of thedielectric material was 0.089%. In this instance, the protrusions havinga diameter of φ 0.25 were formed in each vertex of equilateral triangleshaving a side of 8 mm which were adjacent to each other. The height ofthe protrusions was 10 μm.

According to this electrostatic chuck, it was possible to extremelyreduce the temperature variation which occurs over time at the time ofprocessing a silicon wafer as a material to be clamped due to both ofsmall variation of the surface roughness after the plasma irradiationand the very small contact area with the material to be clamped.

The POPOFF clamping force was 100 torr or more with respect to allsamples. Specifically, it was shown that sufficiently useful force forclamping a silicon wafer or the like was obtained

As is explained in the above, according to the present invention, it ispossible to produce an electrostatic chuck in which the surface can bekept smooth after being exposed to plasma, so as to protect a materialto be clamped such as a silicon wafer from being contaminated withparticles, and which is excellent in clamping and releasing a materialto be clamped.

1. An electrostatic chuck comprising a dielectric material in whichalumina is 99.4 wt % or more, titanium oxide is more than 0.2 wt % andequal to or less than 0.6 wt %, whose average particle diameter is 2 μmor less, and whose volume resistivity is 10⁸-10¹¹ Ωcm in roomtemperature, wherein the electrostatic chuck is used in a lowtemperature of 100° C. or less.
 2. An electrostatic chuck comprising adielectric material in which alumina is 99.4 wt % or more, titaniumoxide is more than 0.2 wt % and equal to or less than 0.6 wt %, whoseaverage particle diameter is 2 μm or less, whose bulk density is 3.97g/cm³ or more, and whose volume resistivity is 10⁸-10¹¹ Ωcm in roomtemperature, wherein the electrostatic chuck is used in a lowtemperature of 100° C. or less.
 3. The electrostatic chuck according toclaim 1 or 2, wherein the dielectric material having a smooth surfacecomprises a plurality of protrusions on which a material to be clampedis mounted, in which the ratio between the total area of the topsurfaces of the protrusions and the area of the surface of thedielectric material is equal to or more than 0.001% and less than 0.5%,and the height of the protrusions is 5-15 μm.